The invention relates to a method for determining the quality of a video signal and/or of a television picture and also to a circuit arrangement for carrying out the method according to the invention.
The reception and reproduction of television pictures in stationary receiving stations poses scarcely any difficulty because the reception conditions are good and remain largely constant. By contrast, the reception conditions for a mobile receiving station may fluctuate considerably depending on the nature of the terrain. If the mobile receiving station is located in a hilly part of the country, for example, echoes may interfere considerably with the reception; in the radio shadow of mountains or hills, the radio link may even collapse completely, with the result that only noise rather than a television picture can be seen on the screen.
In the meantime, motor vehicles, such as e.g. passenger cars and touring coaches, but also railroad vehicles are being equipped with television receivers and screens in order to be able, on the one hand, to display messages, for example traffic messages communicated via teletext, or, on the other hand, to entertain travelers with television programs. The reception conditions in a moving receiving station fluctuate considerably, under certain circumstances, owing to the terrain currently being traveled through; these reception conditions mean that great strain is put on traveling television viewers"" eyes, in an unpleasant manner, because the picture quality can fluctuate to a considerable extent. If the vehicle travels through a radio shadow, for example, all the viewer sees on the screen is noise. Viewing a television program subjected to such interference is more likely to lead to the viewers becoming tired than serve to entertain them.
It is known to improve the reception of radio signals in mobile receiving stations by means of multipath reception, referred to as diversity. Antenna diversity is understood to mean that a receiver can be connected to one of a plurality of antennas, which are usually spatially separated, while frequency diversity designates a system comprising a plurality of receivers which receive identical signals or the same programs at different frequencies. Space diversity is understood to mean a system comprising a plurality of receivers which receive identical signals by means of spatially separated antennas.
In order to be able to select the receiver with the best reception in a diversity reception system having a plurality of receivers, a meaningful criterion is necessary.
The object of the invention, therefore, is to specify a method for determining the quality of a video signal and/or of a television picture.
The invention achieves this object by virtue of the fact that interference pulses occurring in a line after the horizontal sync pulse are detected and evaluated with regard to their parameters in order to obtain a measure of the quality of the video signal.
The invention is based on the first insight that interference pulses occurring in a line after the horizontal sync pulse can serve as a measure of the picture quality. The invention is furthermore based on the second insight that the number of interference pulses per line and also further parameters which characterize the interference pulses, such as amplitude, pulse width and position of the interference pulses, within a line are important physical variables which determine the picture quality.
In accordance with the first exemplary embodiment of the invention, the interference pulses detected in a line are counted. Therefore, the number of interference pulses is a measure of, or to put it better a dimension figure for, the picture quality.
If, for example in a diversity reception system, a plurality of receivers receive television signals of the same program, the number of interference pulses, detected in the individual receivers, for each line represents a dimension figure for the picture quality. It is therefore possible to select the receiver which in each case has the smallest number of interference pulses for each line. By way of example, a counter which counts the interference pulses within a line may be provided for each receiver. The receiver with the smallest number of interference pulses can then be selected in each case as the receiver having the instantaneously best picture.
However, cases may also arise where the receiver with the smallest number of interference pulses per line yields a poorer picture than e.g. another receiver which has more interference pulses per line, because not only the number of pulses but also their amplitude and width influence the picture quality. If, for example, many interference pulses having a small amplitude occur in one receiver, while, on the other hand, the interference pulses have larger amplitudes in the receiver with the smallest number of interference pulses per line, the receiver with the larger number of interference pulses may yield a better picture than the receiver with fewer interference pulses per line, but having a larger amplitude.
In the case of another exemplary embodiment of the invention, therefore, the amplitudes of the interference pulses occurring within a line are added to form a total amplitude, the value of which represents a dimension figure for the picture quality. The amplitudes of the interference pulses can e.g. be added by means of a counter or an integrator.
In a diversity reception system in whose receivers the picture quality is determined in accordance with the method described in claim 3 or 5, it is possible to select e.g. the receiver with the smallest total amplitude as the receiver which is instantaneously yielding the best picture.
The pulse width of an interference pulse has a role similar to the amplitude of an interference pulse. In a diversity reception system, the receiver with the smallest number of interference pulses may yield a poorer picture than another receiver if the pulse widths are larger in the case of the receiver with the smallest number of interference pulses than in the case of the other receiver.
For this reason, in the case of the third exemplary embodiment of the invention the pulse widths of the interference pulses occurring in a line are added to form a total pulse width, for example by means of a counter or an integrator.
In a diversity reception system in which the picture quality of each receiver is determined in accordance with the method according to the invention, the receiver with the smallest total pulse width yields the best television pictures. It is expedient to select this receiver for reception.
In addition to the number, amplitude and width of the interference pulses, the position thereof within a line also critically determines the picture quality. Interference pulses at the beginning and end of a line influence the picture quality to a lesser extent than interference pulses in the central region of the line. In order to take this non-uniform effect into account, the fourth exemplary embodiment of the invention provides for the positions of the interference pulses within a line to be detected and weighted with a factor. A line can, for example, be divided into a plurality of sub-areas, each sub-area being assigned a factor. The number of interference pulses detected in a sub-area is weighted with the factor assigned to this sub-area. The dimension figures obtained by weighting in this way for each sub-area are added to form a total dimension figure, which characterizes the picture quality.
It is expedient to weight interference pulses to a greater extent in the sub-areas where they influence the picture quality or the picture content to a greater extent than in the other areas. As in the case of the other exemplary embodiments, the total dimension figure increases as the picture quality decreases, and a decreasing total dimension figure indicates an increase in the picture quality.
In a diversity reception system it is possible to select the receiver which generates the best picture quality, as in the case of the exemplary embodiments described above.
It is particularly advantageous to combine the first four exemplary embodiments of the invention that have been described up to this point to form a fifth exemplary embodiment.
In the case of the fifth exemplary embodiment of the invention, the interference pulses occurring in a line are counted by means of a counter; the number of interference pulses represents a first dimension figure. A second dimension figure is the sum of the amplitudes, which are added to form a kind of total amplitude. Likewise, addition of the pulse widths of the interference pulses occurring in a line affords a kind of total pulse width, which represents a third dimension figure. Finally, the positions of the interference pulses occurring in a line are additionally detected.
The first, second and third dimension figures and also the positions of the interference pulses are evaluated e.g. in a processor in such a way that the dimension figures and the individual positions of the interference pulses are weighted with factors. The factors are chosen for example in such a way that those parameters which have less influence on the picture quality and the picture content are weighted to a lesser extent than those parameters which have a large influence on the picture quality and the picture content. It is possible to weight only individual parameters; it is alternatively possible, for example, to weight the position, amplitude and pulse width of each interference pulse. The dimension figures that have been weighted in this way are added to form a first total dimension figure, which represents a meaningful measure of the picture quality.
For the sake of completeness, it shall be noted that the amplitudes and pulse widths can be weighted before and/or after their addition. The individual dimension figures and positions of the interference pulses can be evaluated and weighted in a processor, for example. The total dimension figure of one or more lines may be stored in a store.
In the exemplary embodiments that have been explained up to this point, the interference pulses are counted by means of a digital counter. The addition of the weighted or unweighted amplitudes and also of the weighted or unweighted pulse widths is effected using digital technology, e.g. by means of a counter, while an integrator can be provided for this purpose in the case of analog technology. Analog sample-and-hold elements or digital stores may be provided as the stores.
In accordance with another mode of the invention, it is possible, for example, for only those interference pulses whose amplitude and/or pulse width exceeds a predeterminable threshold value to be evaluated.
The individual dimension figures can be combined to form a first total dimension figure for each line. However, it is also possible to form a second total dimension figure for a field or a frame by addition of the individual first total dimension figuresxe2x80x94each line is assigned a first total dimension figure. In this method, the first total dimension figures of the individual lines can be weighted for example in such a way that lines which have a greater influence on the picture quality are weighted to a greater extent than those lines which have only a slight influence on the picture quality.
The invention will now be explained in more detail using the exemplary embodimentsxe2x80x94illustrated in the figuresxe2x80x94of circuit arrangements for carrying out the method according to the invention.